Frank, Mark R.
M.S. (Master of Science)
Department of Earth, Atmosphere and Environment
Porphyry ore deposits are major sources of copper (Cu) and gold (Au) and form when a magmatic volatile phase (MVP) exsolves from a porphyritic intermediate to felsic melt and transports metals to surrounding country rock where they precipitate in an ore zone. There is a consensus that the chloride concentration of the MVP has a direct relationship to the concentration of Cu and Au in the MVP, but the role of dissolved HCl on Cu and Au concentrations has not been addressed. It is hypothesized that increasing HCl concentrations in the MVP will result in higher concentrations of Cu and Au, facilitating the efficient transportation of the metals from the melt to the ore zone. Experiments were conducted in the MVP-bornite-chalcopyrite-pyrrhotite-Au system at 700°C and 100 MPa. These PTX conditions recreate the porphyry environment with an MVP that is an immiscible vapor-brine pair with salinities of 3.8 wt.% NaCleq. and 48 wt.% NaCleq., respectively. Gold capsules were used and were the source of Au, ƒO2 was fixed with a Ni-NiO buffer, and ƒS2was fixed by the sulfide mineral assemblage. HCl concentrations in the MVP were varied with NaCl:HCl molar ratios of 50:1, 25:1, or 15:1 in the MVP; these correspond to 309 μg/g, 611 μg/g, or 1010 μg/g HCl in the MVP, respectively.. Fluids were trapped in synthetic fluid inclusions during the experiment and elemental concentrations of Cu, Au, Fe, Na, K, Zn, Pb, and Ca were measured by using LA-ICP-MS. Iron in the brine was 5030±1300 µg/g and 6700±3700 µg/g and was 323±88 µg/g and 1550 µg/g in the vapor at 309 and 1010 µg/g HCl, respectively. Gold in the brine was 42.1±19 µg/g and 94.5±59 µg/g at 309 and 1010 µg/g HCl, respectively, whereas Au in the vapor was 2.17 µg/g and 24.0±9.7 µg/g at 1010 and 611 µg/g HCl, respectively. Copper in the brine was 645±220 µg/g and 5030±1300 µg/g at 611 and 309 µg/g HCl, respectively, while Cu in the vapor was 87.7±17 µg/g and 670±620 at 309 and 611 µg/g HCl, respectively. Iron follows the hypothesized direct relationship between metal and HCl concentration in both vapor and brine, as does Au in the brine. Gold in vapor and Cu in vapor and brine do not exhibit discernable trends as the data have a greater variability and higher uncertainty. The data of this study suggest that high-salinity and high-HCl fluids in porphyry systems can transport elevated concentrations of Fe, Cu, and Au, conditions most often found associated with granite-diorite melts at depths of 2-6 km, elevated Cl:H2O and high aluminosity.
Lucchesi, Corrie A., "Copper and Gold in Vapors and Liquids as a Function of HCl at 700°C AND 100 MPa" (2020). Graduate Research Theses & Dissertations. 7379.
Northern Illinois University
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